What signifies the onset of immersion hypothermia?

The onset of immersion hypothermia is primarily signified by a drop in body temperature below normal levels. When a person is exposed to cold water, the body loses heat much more rapidly than in air, leading to a decrease in core body temperature. This condition can begin to affect bodily functions, including cognitive and motor skills, as the temperature drops.

Understanding this concept is crucial for recognizing the symptoms and risks associated with prolonged exposure to cold water. While immediate panic and confusion may occur as a result of cold shock response, they are not definitive indicators of hypothermia itself; instead, they usually happen in the very initial moments of immersion. Likewise, while an increase in heart rate may accompany cold immersion due to the body's instinctive reaction, it is not a clear sign of hypothermia. A rise in blood pressure, surprisingly, can occur temporarily but is not relevant to diagnosing immersion hypothermia. Hence, the correct choice focuses on the core physiological change—body temperature reduction—that is the hallmark of this dangerous condition.